A variety of techniques have been used for the deposition of ferroelectric thin films. In general, the thin film deposition techniques can be divided into two major categories: (1) physical vapor deposition (PVD) and (2) chemical processes. Among the PVD techniques, the most common methods used for the deposition of ferroelectric thin films are electron beam evaporation, rf diode sputtering, rf magnetron sputtering, dc magnetron sputtering, ion beam sputtering, molecular beam epitaxy (MBE), and laser ablation. The chemical processes can be further divided into two subgroups, i.e., chemical vapor deposition (CVD), and wet chemical processes including sol-gel process and metalorganic decomposition (MOD).
The PVD techniques require a high vacuum, usually better than 10.sup.-5 torr, in order to obtain a sufficient flux of atoms or ions capable of depositing onto a substrate. The advantages of the PVD techniques are (1) dry processing, (2) high purity and cleanliness, and (3) compatibility with semiconductor integrated circuit processing. However, these are offset by disadvantages such as (1) low throughput, (2) low deposition rate, (3) difficult stoichiometry control, (4) high temperature post-deposition annealing, and (5) high equipment costs.
Laser ablation is a newly developed thin film deposition technique and the understanding of this process is in its infant period. Laser ablation has found some success in depositing high temperature superconducting films. The major problems of this technique are the composition and thickness non-uniformity of the deposited films over a large scale.
The wet chemical processes include MOD and sol-gel process. The advantages of the wet chemical process are (1) molecular homogeneity, (2) high deposition rate and high throughput, (3) excellent composition control, (4) easy introduction of dopants, and (5) low capital cost, since deposition can be done in ambient condition and so no vacuum processing is needed. The major problems due to this wet process are (1) film cracking during the post-annealing process and (2) possible contamination which results in difficulty in incorporating this technique into semiconductor processing. However, because it provides a fast and easy way to produce the complex oxide thin films, this wet chemical process has a very important role in the investigation of the interrelationship among the processing, the microstructure, and the property of the films.
Of all the above mentioned techniques, the metalorganic chemical vapor deposition (MOCVD) technique appears to be one of the most promising because it offers advantages of simplified apparatus, excellent film uniformity, composition control, high film densities, high deposition rates, excellent step coverage, and amenability to large scale processing. The excellent film step coverage that can be obtained by MOCVD cannot be equaled by any other technique. Purity, controllability, and precision that have been demonstrated by MOCVD are competitive with the MBE technique. More importantly, novel structures can be grown easily and precisely. MOCVD is capable of producing materials for an entire class of devices which utilize either ultra-thin layers or atomically sharp interfaces. In addition, different compositions can be fabricated using the same sources.
The first successful deposition of oxide-based ferroelectric thin films by CVD was reported by Nakagawa, et al. in "Preparation of PbTiO.sub.3 ferroelectric thin film by chemical vapor deposition," Jpn. J. Appl. Phys., 21(1), L655 (1982). They deposited PbTiO.sub.3 films on Pt-coated silicon wafers by using TiCl.sub.4, PbCl.sub.2, O.sub.2, and H.sub.2 O as source materials. Several problems arose from their studies: (1) very high evaporation temperature (700.degree. C.) was required of PbCl.sub.2, (2) poor ferroelectric properties (P.sub.r =0.16 .mu.C/cm.sup.2 and E.sub.c =14.5 kV/cm), (3) poor composition uniformity in the bulk of PbTiO.sub.3 films, and (4) crystallographic imperfections due to water and/or chloride contamination. Obviously, chlorides are not ideal precursors for the CVD process. In general, metalorganic precursors have relatively high vapor pressures at lower temperatures. By carefully selecting the organic compounds, the undesirable contaminations in the films can be completely excluded. Metalorganic compounds are now used almost exclusively for the deposition of oxide thin films.
One of the challenges in MOCVD is the choice and synthesis of the precursors. Precursors for MOCVD are chosen on the basis of their ability to satisfy several requirements, including: sufficient vapor pressure at low temperatures, thermal stability at vaporization temperature, rapid decomposition with minimal ligand contamination at low substrate temperatures, stability under ambient conditions, and minimal toxicity.
Vapor pressure of 0.1-0.5 torr at a vaporization temperature of less than 170.degree. C. is necessary for effective deposition; low vaporization temperature provides efficiency of heating. Efficient transportation of the precursor from the bubbler to the substrate necessitates stability to prolonged heating in vacuo at the vaporization temperature. Thermal stability ensures minimal decomposition or intermediate reactions in the bubbler, or in gas phase before contacting the substrate. Low deposition temperatures of 350.degree.-500.degree. C. ensure minimal film-substrate interactions, and efficiency of heating. The decomposition of precursors at these temperatures should be rapid and clean, with minimal contamination from organic constituents of the ligands such as hydrogen, carbon or fluorine. Finally, a precursor's stability to oxygen and moisture, and its non-toxicity are necessary for ease of handling. Hygroscopic, air sensitive or toxic precursors require special closed chambers for transportation and storage. Additionally, hydration or oxidation can render the precursor nonreactive.
Classes of useful precursors for MOCVD of PbO include: (1) lead alkyls, (2) lead alkoxides, and (3) lead .beta.-diketonates. The high volatility of the liquid lead alkyls such tetraethyl lead and lead alkoxides such as lead ethoxide, decreases ease of handling due to the extreme toxicity of these compounds. Additionally alkoxides are extremely sensitive to moisture, and consequently decompose via autohydrolysis.
An alternative of precursors is the lead .beta.-diketonates. Several precursor requirements are fulfilled by the lead .beta.-diketonates: they are not volatile at room temperature and are therefore less toxic than the alkyls and alkoxides, they have sufficient vapor pressure of 0.1-0.5 torr below 200.degree. C., and they decompose at temperatures less than 500.degree. C. See R. E. Sievers and J. W. Connolly, "Tris (1, 1, 1, 2, 2, 3, 3-Heptafluoro-7, 7-dimethyl-4, 6-octadionato) Iron (III) and Related Complexes", Inorganic Synthesis, 12, XII, p. 72-77 (1969). Lead .beta.-diketonates are of the form: ##STR1##
The R.sub.1 and R.sub.2 groups are alkyl or fluorinated alkyl groups. The properties of the different lead .beta.-diketonate species are a function of the nature of the R-substituents. For instance, lead acetylacetonate (R.sub.1 =R.sub.2 --CH.sub.3) is stable to air and moisture. Volatility is also a function of the nature of the R-group; lead acetylacetonate has 1 torr vapor pressure at 95.degree. C., lead trifluoroacetylacetonate (R.sub.1 =CH.sub.3, R.sub.2 =CF.sub.3) has 1 torr vapor pressure at 72.degree. C., and lead hexafluoroacetylacetonate (R.sub.1 =R.sub.2 =CF.sub.3) has 1 torr vapor pressure at 67.degree. C.[4]. Additionally, the R-substituents affect the strength of the metal-oxygen bond; consequently, decomposition temperature is a function of the R-group. This phenomenon is discussed for Cu and Fe .beta.-diketonates with various R-substituents in C. Reichert and J. B. Westmore, "Mass Spectral Studies of Metal Chelates, VI, Mass Spectra and Appearance Potentials of .beta.-diketonates of Copper (II)", Canadian Journal of Chemistry, 48, pp. 3213-3222 (1970), and H. F. Holtzclaw, R. L. Lintvedt, H. E. Baumgarten, R. G. Parker, M. M. Bursey, P. F. Rogerson, "Mass Spectra of Metal Chelates, I, Substituent Effects on Ionization Potentials and Fragmentation Patterns of some 1-methyl-3-alkyl-1, 3-dione Copper (II) Chelates", Journal of the American Chemical Society, 91, 14, pp. 3774-3778.
Lead bis-heptafluorodimethyloctadione, or Pb(fod).sub.2, is a lead .beta.-diketonate which is potentially a useful precursor for MOCVD of PbO; it has been deposited at 450.degree.-600.degree. C. See D. C. Bradley, "Metal Alkoxides as Precursors for Electronic and Ceramic Materials", Chemical Reviews, 89, 6, pp. 1317-1322 (1989). The R-substituents for Pb(fod).sub.2 are R.sub.1 =C.sub.3 F.sub.7 and R.sub.2 =C(CH.sub.3).sub.3, it has sufficient vapor pressure of 1 torr at 95.degree.-105.degree. C., and it is stable to air and moisture. See R. C. Mehrotra, R. Bohra, D. P. Gaur, "Metal .beta.-diketonates and Allied Derivatives, Chapter 2", Academic Press, Inc., New York, NY, 1978. A similar compound is lead bis-tetramethylheptadione, or Pb (thd).sub.2. The two compounds, Pb(fod).sub.2 and Pb(thd).sub.2 differ only by substituents on their respective ligands. Properties such as volatility, thermal stability, moisture stability and temperature of decomposition differ as a function of the substituents. Pb(fod).sub.2 has a vapor pressure of 1 torr at 98.degree. C. while Pb(thd).sub.2 has a vapor pressure of 1 torr at 60.degree. C. Both are relatively stable and safe for handling under ambient conditions. See G. K. Schwitzer, B. P. Pullen and Y. Fang, Anal. Chim. Acta, 43, 332-334 (1968).
References on MOCVD using .beta.-diketonates include U.S. Pat. No. 5,096,737 by Baum, et al. and U.S. Pat. No. 5,045,348 by Brierley, et al.